The present invention relates to the field of turbomachine blades, and more particularly to the field of turbomachine rotor blades.
In the present context, the term “turbomachine” is used to designate any machine in which energy can be transferred between a fluid flow and at least one set of blades, such as for example, a compressor, a pump, a turbine, or a combination of at least two of them. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the fluid through the turbomachine.
Such a turbomachine may comprise a plurality of stages, each stage normally comprising two sets of airfoils, namely a set of movable blades and a set of stationary guide vanes. Such a set comprises a plurality of airfoils that are offset from one another in a lateral direction. Typically the airfoils are arranged radially around a central axis A. Thus, such a set forms a rotor when it constitutes a set of moving blades, or a stator when it constitutes a set of guide vanes. The proximal end of each blade relative to the central axis A is normally referred to as its root, while the distal end is normally referred to as its tip. The distance between the root and the tip is known as the blade height. Between its root and its tip, the blade is made up of a stack of aerodynamic profiles extending substantially perpendicularly to a radial Y axis. In this context, the term “substantially perpendicular” means that the plane of each profile may present an angle relative to the radial Y axis that is close to 90°, e.g. lying in the range 60° to 120°.
In such a turbomachine, such a rotor is normally surrounded by a casing. In order to limit flow losses in the rotor, it is typically desirable to limit the radial clearance between the blade tips and the inside walls of the casing. Nevertheless, this reduction in radial clearance gives rise to an increase in the risk of contact between the blade tips and the inside walls of the casing. Such contact can be particularly dangerous if the frequency of contact enters into resonance with the resonant frequency of the blades in bending. This problem has thus given rise to studies such as that described by Kevin E. Turner, Michael Dunn, and Corso Padova under the title “Airfoil deflection characteristics during rub events” in “ASME Turbo Expo 2010: Power for land sea and air”, Jun. 14-18, 2010, Glasgow, UK, and that described by Robin J. Williams under the title “Stimulation of blade casing interaction phenomenon in gas turbines resulting from heavy tip rubs using an implicit time marching method”, in “ASME Turbo Expo 2011”, Jun. 6-10, 2011, Vancouver, British Columbia, Canada.
Simultaneously, for aerodynamic reasons, in order to increase the efficiency of compressors and fans, and in particular those reaching transonic flow speeds, forwardly-swept blades have been proposed, e.g. in US patent application US 2010/0054946 A1, and also in the study entitled “Influence of sweep on the 3D shock structure in an axial transonic compressor”, described by Jörg Bergner, Stephan Kablitz, Dietmar K. Hennecke, Harald Passrucker, and Erich Steinhardt in ASME Turbo Expo 2005: Power for land sea and air”, of Jun. 6-9, 2005, at Reno-Tahoe, Nev., USA.
Nevertheless, for geometrical reasons, forwardly-swept blades can present behavior that is particularly negative in the event of making contact with the casing. As a result of their shapes, the lateral deflection caused by such contact can worsen the phenomenon of dynamic self-engagement.
French patent application FR 2 851 798 A1 discloses a rotor blade having a distal segment that is swept back between the blade tip and an intermediate segment that presents forward sweep over a large portion of the blade height. The backward sweep of the distal segment in such a configuration may serve to reduce the risk of dynamic self-engagement, at least in part.